1. Field of the Invention
The present invention relates to a battery including a cathode, an anode, and an electrolytic solution.
2. Description of the Related Art
In recent years, portable electronic devices such as camera-integrated video tape recorders (VTR), mobile phones, and laptop personal computers have become widely popular. There is strong demand for reduction in size and weight and increase in life span of the portable electronic devices. In accordance with the demand, batteries, particularly light-weight secondary batteries having high energy density, are being developed as a power source for the portable electronic devices.
Most promising among the secondary batteries are a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) and a secondary battery using deposition and dissolution of lithium (so-called lithium metal secondary battery), because higher energy density is achieved compared to a lead battery and a nickel cadmium battery.
The secondary batteries described above have an electrolytic solution including a solvent and an electrolyte salt dissolved in the solvent. A mixed solvent that is a mixture of a high-permittivity solvent and a low-viscosity solvent is widely used as the solvent. High-permittivity solvents include, for example, ethylene carbonate and propylene carbonate that easily solvate with the electrolyte salt. Low-viscosity solvents include, for example, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate that are superior in ionic conductivity. Regarding a composition of the electrolytic solution, solvent type, solvent mixture ratio, electrolyte salt type, electrolyte salt concentration, and the like are adjusted to improve electric conductivity.
When safety of the secondary battery is taken into account, in addition to electric conductivity, withstand voltage is an important characteristic demanded for the electrolytic solution. In other words, it is important that the secondary battery sufficiently endure even a state in which a high voltage and a high current exceeding standard values are applied.
Possible cases in which the high voltage and high current are applied are, for example: the case that a power supply circuit or a charger fails; the case that an electric charge that is equal to or more than a predetermined amount is applied as a result of misuse by a user, causing the secondary battery to become overcharged; and the case that a high current is applied because of an external short circuit, an internal short circuit, and the like. In the cases described above, the electrolytic solution is decomposed on an electrode surface, so gas and decomposition heat are generated. In this case, if the gas and the decomposition heat are continuously generated, the secondary battery may explode or ignite. In particular, lithium/transition metal complex oxides that are widely used as a cathode active material are known to become unstable oxides and release oxygen when the secondary battery is overcharged (for example, refer to Solid State Ionics 69, J. R Dahn, et al., 1994, page 265).
The electrolytic solution including the mixed solvent that is the mixture of the high-permittivity solvent and the low-viscosity solvent is advantageous for improvement of electric conductivity. However, because oxidation resistance is low, oxidative decomposition of the electrolytic solution easily occurs on the electrode surface when the secondary battery is overcharged. In this instance, when a lithium/transition metal complex oxide is used as the cathode active material, the secondary battery ignites with extreme heat generation, thereby increasing risk of so-called thermorunaway.
A technique in which an overcharge inhibitor is included in the electrolytic solution to improve safety when the secondary battery is overcharged, even if the above-described mixed solvent is used is known. Overcharge inhibitors include, for example, benzene, cyclohexylbenzene, t-pentylbenzene, terphenyl, anisole derivatives, biphenyl, 4,4′-dimethylbiphenyl, 3-R-thiophene, 3-chlorothiophene, furan, and non-ionic aromatic compounds including 2,2-diphenylpropane or an alkyl group (for example, refer to Japanese Unexamined Patent Application Publication Nos. H07-302614, H09-106835, H09-171840, H10-321258, H10-275632, H11-162512, 2000-156243, 2001-023690, 2003-022838, 2005-032701, and Japanese Patent No. 2939469).
A technique in which the overcharge inhibitor is included in the cathode instead of in the electrolytic solution is also known (for example, refer to Japanese Patent No. 3778805). In the technique, terphenyl is mixed with the cathode active material and included in a cathode active material layer.